(1) Field of the Invention
The present invention relates of a control technique of an optical wavelength variable filter to be utilized in various equipments for optical communication, in particular, to a control method and a control apparatus for controlling wavelength characteristics of a band rejection type optical wavelength variable filter.
(2) Related Art
There have been demanded developments of optical communication systems and optical signal processing systems capable of constructing networks of large capacities and ultra-long distance with an explosive increase of IP data communication demand. In a transmission system adopting a wavelength-division multiplexing (WDM) transmission as a basic technique thereof, it is possible to realize the large capacity transmission and easily perform the division-multiplication with the wavelength as a unit, so that the construction of flexible optical networks that performs multiplication-division of different kinds of services at optical levels, such as, optical cross-connecting (OXC), optical add/drop multiplexing (OADM) and the like. Therefore, the development and manufacturing of transmission apparatus and signal processing apparatus using the above system have been remarkably made.
In these apparatuses, there are used many optical function devices, such as, an optical wavelength filter that separates a signal light for each wavelength, and the like. Specifically, the optical wavelength filter is used, for example, for the wavelength switching in the OXC and OADM, the separation of each wavelength at a receiving section, the ASE cutting, and the like.
By forming the above mentioned optical wavelength filter on a substrate made of SiO2, LiNbO3 and the like, it becomes possible to achieve the high functions, down-sizing, integration, reduction of electricity, and reduction of cost. Further, in a case where a plurality of optical wavelength filters are cascade connected, it is possible to achieve the narrow transmission band and the improvement of suppression ratio between other channels during used as band-pass type optical wavelength filters, and also it becomes possible to achieve the improvement of extinction ratio during used as band-rejection type optical wavelength filters (notch filters).
However, in such optical wavelength filters adopted with multi-staged structure, there is caused a problem in that the wavelength setting at respective stages needs to be controlled with high accuracy, since filter characteristics fluctuate due to a change in temperature or a change with time lapse. Further, since there is a possibility that the wavelength of light to be input to the optical wavelength filters fluctuates due to a change in environment, the function for controlling the wavelength setting as mentioned above is indispensable to the optical wavelength filters.
In particular, in the optical wavelength filter of band rejection type to be used in the OXC, OADM and the like, if the light of wavelength that should have passed through is erroneously blocked, the services to the users are suspended. An occurrence of such a situation should be avoided. In order to avoid such a situation, a wavelength controlling technique for variably controlling the wavelength (selected wavelength) of light to be blocked from passing through at each stage, with high accuracy and reliability, shall be important.
The present invention has been achieved in view of the above problems, and an object of the present invention is to provide, in an optical wavelength variable filter of band rejection type structured in multi-stages by cascade connecting a plurality of optical filter sections, a control method and a control apparatus for controlling with high accuracy and reliability selected variable wavelengths at respective stages, to obtain desired wavelength characteristics.
In order to achieve the above object, the present invention provides a control method of wavelength characteristics of an optical wavelength variable filter of band rejection type including a plurality of optical filter sections each blocking a light corresponding to a selected wavelength from passing through, the optical filter sections being cascade connected to be in a multi-staged structure, wherein, when the optical wavelength variable filter has wavelength characteristics formed with a blocking band including a wavelength band according to a deviation amount obtained by deviating respective selected wavelengths from one another of the plurality of optical filter sections, a dropped light to be blocked from passing through at the optical filter section corresponding to the selected wavelength most closest to the center wavelength of the blocking band, among the respective selected wavelengths, is taken out to be monitored, and the respective selected wavelengths are adjusted based on a peak wavelength of the monitored dropped light, to control the wavelength characteristics.
Further, according to the present invention, a control apparatus of wavelength characteristics of an optical wavelength variable filter of band rejection type including a plurality of optical filter sections each blocking a light corresponding to a selected wavelength from passing through, the optical filter sections being cascade connected to be in a multi-staged structure, wherein, when the optical wavelength variable filter has wavelength characteristics formed with a blocking band including a wavelength band according to a deviation amount obtained by deviating respective selected wavelengths from one another of the plurality of optical filter sections, said control apparatus comprises a monitoring section that takes out to monitor a dropped light to be blocked from passing through at the optical filter section corresponding to the selected wavelength most closest to the center wavelength of the blocking band, among the respective selected wavelengths, and a controlling section that adjusts the respective selected wavelengths based on a peak wavelength of the dropped light monitored by the monitoring section, to control the wavelength characteristics.
According to the control method and control apparatus of an optical wavelength variable filter as mentioned above, in the band rejection type optical wavelength variable filter of multi-staged structure, the dropped light to be blocked from passing through at the optical filter section corresponding to the selected wavelength most closest to the center wavelength of the blocking band among the selected wavelengths at respective stages is taken out to be monitored, so that the peak wavelength of the dropped light can be detected stably. Thus, it becomes possible to control the selected wavelengths at the respective optical filter sections with high accuracy and reliability. In this way, even if the setting of filters or the wavelength of input light fluctuates under an influence of a change in temperature, a change with time lapse or the like, the selected wavelengths at respective stages are adjusted following such a fluctuation, and thus it becomes possible to obtain desired wavelength characteristics stably.